1. Field of the Invention
The present invention relates to a semiconductor laser used for optical information apparatuses such as optical disk memories, laser printers, etc., and for optical communication, and more particularly, to a semiconductor laser of a visible ray region having a lasing wavelength ranging from 0.6 to 0.7 .mu.m.
2. Description of the Related Art
There has been a great demand for semiconductor lasers having a short lasing wavelength in the field of optical information systems. Semiconductor lasers of a visible ray region having a lasing wavelength ranging from 0.6 to 0.7 .mu.m have already been produced.
FIGS. 4A to 4C show a process for making a conventional InAlGaP semiconductor laser which is generally known as a semiconductor laser of a visible ray region whose lasing wavelength ranges from 0.6 to 0.7 .mu.m.
As shown in FIG. 4A, using metal organic chemical vapor deposition (MOCVD) techniques, an n-type GaAs buffer layer 2, an n-type In.sub.0.5 (Ga.sub.1-x Al.sub.x).sub.0.5 P clad layer 3 (0.4.ltoreq..times..ltoreq.1.0), an undoped In.sub.0.5 (Ga.sub.1-y Al.sub.y).sub.0.5 P active layer 4 (0.ltoreq.y.ltoreq.0.2), a p-type In.sub.0.5 (Ga.sub.1-x Al.sub.x).sub.0.5 P clad layer 5, and an n-type GaAs current block layer 6 are sequentially formed on an n-type GaAs substrate 1 serving as a semiconductor substrate.
As shown in FIG. 4B, the n-type GaAs current block layer 6 is selectively removed by photoetching to form a mesa-shaped stripe groove so as to expose part of the surface of the clad layer 5.
As shown in FIG. 4C, a p-type GaAs cap layer 7 is formed on the substrate by the MOCVD technique after the mesa-shaped groove is provided.
After the layer 7 is formed, the GaAs substrate 1 is lapped so that a thickness of the structure is about 80 to 90 .mu.m. A p-type electrode (An/Zn) and an n-type electrode (Au/Ge) are then provided on the GaAs cap layer 7 and GaAs substrate 1, respectively. After the wafer is cleaved to provide semiconductor chips, the chip is mounted on a heat sink through the p-type electrode, and then packaged.
On a practical level, the conventional InAlGaP semiconductor lasers have a lasing wavelength of 670 to 690 nm, a light output of 3 to 5 mW, and a life time of 10,000 hours or more. On an investigation level, the semiconductor lasers have a continuous-wave operation (.about.630 nm) at a room temperature and a maximum light output of 20 to 30 mW (680 nm).
GaAlAs semiconductor lasers having a lasing wavelength of 780 to 850 nm provide the light output of 30-40 mW on the practical level and that of several hundreds mW on the investigation level.
From the above viewpoint, the InAlGaP semiconductor lasers will be required to have the light output of several tens mw at the most advantageous lasing wavelength of 630 to 650 .mu.m in the visible ray region.
When both an Al mixed crystal ratio x of the active layer 4 and an Al mixed crystal ratio y of the n-type clad layer 3 and p-type clad layer 5 are increased, and the light output is increased in order to lower the lasing wavelength of the semiconductor laser, there occurs such a problem that the optical destruction level is lowered at the emitting end surface of the laser beam. The reason is as follows. Since the light density is increased with the increase in the current density injected into the active layer, the emitting end surface of the semiconductor laser approaches the limit of crystal destruction. The limit tends to be lowered as the Al mixed crystal ratio of the lasing region is increased. Such a destruction may easily occur at the end portion exposed to air as compared with the internal portion of the crystal.
The following shows techniques for improving the destruction level of the emitting end surface caused by such a high output operation:
1) The emitting area at the emitting end surface is increased by the structure of epitaxial growth layers to decrease the light density per unit area.
2) The end surface is covered with a transparent protection film to prevent it from being directly exposed to air.
3) The composition of crystal near the emitting end surface is changed, or an electrode structure is provided so that a current does not flow through the emitting end surface, that is, a window structure is provided.
There are also provided the following techniques for forming the window structure:
1') When an active layer, clad layers, etc., are grown on the substrate to obtain a double hetero structure, as shown in FIGS. 5A and 5B, a step or terraced portion is formed at both ends of a substrate 8, on which an n-type buffer layer 9, an n-type clad layer 10, an active layer 11, a p-type clad layer 12, an n-type current block layer 13, and a p-type cap layer 14 are successively grown. Thus, the step is provided at emitting end surfaces of the active layer 11 to render semiconductor layers each having a different composition at end portions.
2') As shown in FIGS. 6A and 6B, an n-type buffer layer 9, an n-type clad layer 10, an active layer 11, a p-type clad layer 12, an n-type current block layer 13, and a p-type cap layer 14 are successively grown on a substrate 8 to obtain a normal double hetero structure. The both ends of the structure are subjected to an impurity diffusion or ion implantation to provide an insulating layer 15 thereat. It is thus possible to prevent the current injection and the lasing action at the end portions.
3') As shown in FIGS. 7A through 7C, an n-type buffer layer 9, an n-type clad layer 10, an active layer 11, a p-type clad layer 12, an n-type current block layer 13, and a p-type cap layer 14 are successively grown on a substrate 8 to obtain a normal double hetero structure. The both ends of the structure are removed by photoetching using a resist mask 16, and a high resistance layer (window layer) 17 having a band gap energy higher than that of the active layer 11 is provided at the removed portions.
However, in the above technique 3'), when the window layers are grown again, an interface of the lasing action may be easy to have a discontinuous surface and accordingly unwanted defect may occur. The semiconductor laser may be therefore deteriorated.
In the above technique 2'), the impurity-doped region may be damaged by thermal diffusion of high impurity concentration or the ion-implantation. Therefore, the semiconductor laser may be deteriorated in the vicinity of the interface of the lasing action.
Although the structure in the above technique 1') does not include the discontinuous surface that is provided by the buried layer as shown in the technique 3'), the crystallinity of inclined portions of the active layer 11 near the emitting end surfaces is generally unsatisfactory and the defect is easy to occur between the inclined portions and flat portions of the active layer.
It is effective that the destruction level at the end surfaces of the semiconductor laser is improved by forming the window layer in the end surfaces. Actually, the above-described techniques of forming the window structure have the respective problems, and a practical semiconductor laser having the window structure has not yet been produced. In particular, materials containing aluminum, such as GaAlAs and InAlGaP, adversely affects the crystallinity of the active layer in the process of making the window layer.
The above-described window structure for improving the optical destruction level in the end surfaces of a semiconductor laser has a drawback in reliability in the process of crystal growth.